The human genome was extracted from chewing gum 5700 years ago.

Modern chewing gum, usually containing polyethylene plastic, can last for decades or even hundreds of years, and may even be longer under appropriate conditions. Some of the first chewing gums made of birch tar and other natural substances have been preserved for thousands of years, including the Stone Age chewing gum unearthed in Denmark 5,700 years ago.

For archaeologists, the life of this sticky substance can help piece together the life of ancient people who chewed tar. The ancient birch chewing gum in Scandinavia preserved enough DNA to reconstruct the complete human genome of its ancient chewer, identify the microorganisms living in her mouth and even reveal the menu of prehistoric food.

"These birch asphalt chewing gums are a bit special in terms of DNA preservation. This surprised us, "said Hanaz Schroeder, a co-author and a molecular anthropologist at the University of Copenhagen. "It is as well preserved as some of the best rock (skull) bones we have analyzed. They are the holy grail of ancient DNA preservation."

Birch pitch is made by heating bark, and it is widely used as prehistoric glue in Scandinavia to fix stone tools on handles. There are usually teeth marks when they are found. Scientists suspect that people chew it for several reasons: it can be extended again after cooling to relieve toothache, because it has mild antibacterial effect, cleans teeth and relieves hunger pain, or simply because they like it.

The water resistance of chewing gum helps to preserve the DNA inside, and its mild antiseptic property helps to prevent microbial decay. But this discovery may also be due to the conditions of the site named Syltholm on an island in southern Denmark, where the thick soil perfectly preserves a large number of unique Stone Age cultural relics. In 20 12, the site began to be excavated, and it was ready to build a tunnel, which provided a unique opportunity for the Roland worster Museum to do archaeological field work.

Silsom has not found human remains, except for the natural communication described by Schroeder and his colleagues today.

This discarded chewing gum provides a lot of information about the chewing device of 5700 years ago. She is a woman, although her age is unknown, but considering that similar birch gum in that era usually bears the imprint of children's teeth, she may still be a child.

From DNA, researchers can begin to piece together some physical characteristics of ancient women and make some inferences about the world she lives in. Schroeder said: "We are sure that she has an amazing combination of black skin, black hair and blue eyes." . "It's interesting, because it's the same combination of physical features, which is obviously very common in Mesolithic Europe. Therefore, all other ancient (European) genomes we know, such as Labrana in Spain, have such a combination of physical characteristics, which is not common in Europe today. Nowadays, the skin color of indigenous Europeans is getting lighter and lighter, but it was obviously not the case before 5000 to 10000 years ago.

This is the artist's explanation of the way Scandinavians chew this ancient chewing gum. When people who chew gum settle in Scandinavia, their family relationships may also help to map people's migration.

Schroeder said: "In fact, her genetic relationship with people in Belgium and Spain is closer than that with people in Sweden, which is only a few hundred kilometers away from the north, telling us how southern Scandinavia lived in the first place." "And it looks like it's from the mainland." This explanation will support the research that after the ice sheet retreated from 12000 years ago to 1 1000 years ago, two different people colonized Scandinavia along the coast of Norway today through a south line and a northeast line.

The individual is a part of the world, and the world is constantly changing with the migration of Nordic groups. Jan Stor, a skeletal archaeologist at Stockholm University? ) said: "We may think that this process, especially in the late Middle Stone Age, is a complex process, with different groups from the South, the West and even the East. They move at different times, sometimes intertwined, and at other times they may remain isolated. " , said by email:

Additional archaeological work shows that this era is a transitional period. Flake stone tools and T-shaped staghorn axes were replaced by polished flint products, pottery and domestic animals and plants. Whether the change to agriculture in this area is a change in the lifestyle of local hunters or the arrival of agricultural immigrants is still a controversial issue.

"This should be the time when agriculture has arrived, with the change of lifestyle, but we can't find traces of farmers' ancestors in her genome, which is quite easy because it originated in the Near East. Therefore, even 5,700 years ago, when other parts of Europe, such as Germany, had agricultural populations of other ancestors, she still looked like a hunter-gatherer in the West, just as people saw it thousands of years ago, "Schroeder said.

"It is very interesting that Neolithic farmers lacked gene flow during this period," added Stoll, who was not involved in the study. "There may be agricultural groups in this area, and they will interact with hunting and gathering groups."

The poor oral hygiene in this era adds more evidence to this series of investigations, because the genetic fragments of food can also be identified in chewing gum.

Maybe not long ago, this woman threw away chewing gum, ate hazelnuts and ducks, and left her own DNA sequence. "Dietary evidence, ducks and hazelnuts, will also support the view that she is a hunter-gatherer and lives on wild resources," Schroeder said, pointing out that the site is full of physical relics, showing dependence on wild resources such as fish, rather than domesticated plants or animals.

"It seems that there may be some hunter-gatherers still living in these places, or living side by side with farmers for hundreds of years," he said.

Scientists have also found countless traces of microorganisms living in women's mouths. Ancient DNA samples always contain microbial genes, but they usually come from the environment. The research team compared the classification and composition of well-preserved microorganisms with those found in modern people's mouths and found that they were very similar.

The researchers were satisfied that the genetic characteristics of ancient oral microorganisms were preserved in women's chewing gum, and studied certain kinds of bacteria and other microorganisms. Most of them are common flora, just like the flora still existing in most people's mouths. Others are outstanding, including bacterial evidence of gum disease and Streptococcus pneumoniae, which can cause pneumonia today, causing 65,438+0,000 or more infant deaths every year. More than 90% of humans carry

Epstein-Barr virus also exists in this woman's mouth. This virus is usually benign and may be related to serious diseases such as infectious mononucleosis, Hodgkin's lymphoma and multiple sclerosis. Ancient examples of these pathogens can help scientists reconstruct the origin of some diseases and track their evolution over time, including what factors may make them more dangerous.

"In this research, what I am really interested in is microbial DNA," Anders Gershostrom, a molecular archaeologist at Stockholm University, said in an email. "DNA from ancient pathogens is promising, and this frankincense may be a better data source than ancient bones or teeth."

Natalija Kashuba, an archaeologist at Uppsala University in Sweden, and his colleagues also extracted human DNA from several people's ancient birch gum at a site with a history of 10000 years on the west coast of Sweden. "We can start studying this material, which is really interesting, because from the Stone Age to the Iron Age, this material is everywhere in Scandinavia," she said, adding that in places where birch trees are prevalent, including eastward to Russia, gum may survive, and the Scandinavian immigration tide is thought to have originated there. Kashuba said:

It is not entirely because of luck that the discarded cultural relics survived and revealed so much information about the past. She said: "I think we must thank archaeologists, who not only preserved these gums, but also suggested that we might try to deal with them." . "If it weren't for them, I'm not sure most geneticists would be bothered by this material." "